1987 Mercedes Benz 420sel



May, 2, 2009 AT 3:26 PM

Engine Performance problem
1987 Mercedes Benz 420sel V8 Two Wheel Drive Automatic 206000 miles

My car all of a sudden has started to idle real high. In the range of 1500 to 2000 rpms. I noticed the thermal vacume valve was broke and then I started to check all the other vacume lines and they seemed to be ok. I replaced the thermal vacume valve and it still idles high. And the same time that happened the anti-lock brack light has come on and the electric fan stays on now. I purchased a thermal switch for that as well and will replace that too. But the idle is still way to high. What could it be. Thanks Derrick

2 Answers



May, 4, 2009 AT 7:30 AM

Please be patient, I have sent your link to a M/B tech, he should be with you shortly



May, 4, 2009 AT 12:13 PM

I can tell your that the fan staying on the the idle issue are more than likely related. Your thermo valve is the origin of your issue and since it has been replaced I would be suspect of other components in this vacuum circuit. This circuit will not only affect fan operation but also idle speed as it is connected to the distributor, so you are on the right track. I woulld recommend checking the purge system for operation and vacuum line condition. The purge valve is located in the vacuum line between the charcol canister and the throttle valve housing, a control line is routed from the throttle valve housing through the thermo valve and on to the purge valve. Below is a diagram of the system:


It may be a bit hard to see put this in the image but this is the fuel evap system for your vehicle. To check the operation of this system do the following:
1. Dsiconnect the hose between the purge valve and the canister at the canister. No vacuum should be present at idle. The vacuum should gradually increase with engine speed
2. If no vacuum was present, trace the vacuum line back to the throttle valve housing. Blow through the connection on the housing and ensure the line is clear
3. If vacuum is still not present, remove the white/purple/black hose from the purge valve. If vacuum is now present replace the purge valve. If there is no vacuum the thermo valve is not operating.

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